The sun is the largest and the most important source of energy for the earth. We can use solar energy to prepare hot food using a solar oven. Solar ovens or solar cookers are devices that make use of the solar energy as their fuel, to cook or heat food. Solar ovens make great projects for science fairs. A working model of a solar oven can be made using readily available items such as shoebox, aluminum foil, and other materials. (See ref. 1 & 2)
Paint the entire surface area of the shoebox, both inside and out, with black paint using a brush. Or you may paste black construction paper over the entire surface. A black body is a very good absorber of heat radiation and will facilitate faster cooking in the solar oven.
Open the shoebox lid fully and place the lid on any flat surface, such that the underside of the lid is now facing you. This will provide support for the next step.
Draw a rectangle, with a ruler, that is at an equal distance of 1 inch from each edge of the lid. With a sharp knife, cut through the two short sides and one long side of the rectangle. Gently push the flap up to form an opening on the lid.
Cover the opening in the lid completely with plastic wrap. This plastic covering is the window for the sun's rays to enter your solar oven. It is very important that the plastic covering is strong and does not have any punctures in it. Use a glass covering instead of plastic if your area of residence is particularly windy. Completely tape the edges of the glass sheet to the shoebox with duct tape. Make sure you don't leave any sharp glass edges exposed.
Measure the sides of the underside of the shoebox lid, the underside of the flap, cut from the lid, and of all the sides and the bottom of the inside of the shoebox. Cut aluminum foil to the measurements taken. Now apply glue to inside of the box and the bottom of the lid and paste aluminum foil rectangles over all the surfaces. The inside of the shoebox and the bottom of the lid must be completely covered in aluminum foil (shiny side out); make sure no cardboard is visible. Smooth out all wrinkles and creases in the pasted aluminum foil.
Pull the flap that we cut off the lid upwards, to reveal the underside which has been lined with aluminum foil. Because of the aluminum foil's mirror finish, this flap will work as a reflector, reflecting the sun rays into the shoebox through the window. Adjust the position of the reflector by moving it upwards or downwards, until you reach a position where it reflects maximum amount of sun rays into the shoebox.
Test out your project. Place a cup of water inside the oven, close the lid tightly and place the oven in a sunny area. After around 30 minutes, check the temperature of your water to see if your oven is working.
TL;DR (Too Long; Didn't Read)
Periodically check whether intense sunlight is falling on the oven. The sun moves and you will have to move the oven with it.
The contents inside the oven will get very hot, always use a pair of microwave gloves.